1. Field of the Invention
The present invention relates to flexible printed circuit boards (FPCBs) and, particularly, to a multilayer printed circuit board having electromagnetism screen and waterproof performance.
2. Description of Related Art
With the development of integration of portable electronic devices, multilayer printed circuit boards have been widely used to maintain an electrical connection between a number of components in portable electronic devices.
The multilayer printed circuit board is composed of two or more single-layer printed circuit boards and one or more adhesive layer disposed between the adjacent single-layer printed circuit boards. The multilayer printed circuit board can be manufactured by laminating the two or more single-layer printed circuit boards together with one or more adhesive layer disposed between adjacent single-layer printed circuit boards. After the two or more single-layered printed circuits and one or more adhesive layer are laminated together, two opposite outmost electrical traces are formed using a series of wetting processes, such as cleaning, developing, etching, electro-plating, etc. In the multilayer printed circuit board, each single-layer printed circuit board is composed of an insulation film and a copper film formed on the insulation film. Regarding an outmost single-layer printed circuit board of the multilayer printed circuit board, the copper film of the outmost printed circuit board is required to be made into the outmost electrical trace.
For example, a process for manufacturing the outmost electrical trace of the FPCB includes the following steps. Firstly, the copper film is cleaned in a cleaning solution to remove surface oils of the copper film. Secondly, a photo-resist layer is formed on the surface of the copper film. Thirdly, the photo-resist layer is exposed to light beams using a mask having a predetermined pattern. Thus, one portion of photo-resist layer is covered by the mask, and the other portion of photo-resist layer is not covered and irradiated by the light beams. When the photo-resist layer is made of a positive photo-resist, the uncovered photo-resist layer (i.e. the exposed photo-resist layer) is soluble in a developing agent. Fourthly, the copper film having the photo-resist layer is developed in the developing agent. During the developing process, the exposed photo-resist layer is dissolved in the developing agent, and the residual photo-resist layer forms a patterned photo-resist layer. Thus, one portion of the copper film is covered by the pattered photo-resist layer, and the other portion of the copper film is exposed to the outside. Fifthly, the copper film not covered by the photo-resist layer is dissolved by an etching solution. As a result, the residual copper film covered by the photo-resist layer forms a copper trace. Finally, the photo-resist layer covering the copper trace is eliminated, thereby obtaining the desired outmost electrical trace of the multilayer printed circuit board.
In the above mentioned process, the multilayer printed circuit board is exposed in liquid solutions (e.g., the cleaning solution, the developing agent, the etching solution) repeatedly, and the liquid solution may inevitably penetrate into the multilayer printed circuit board. In other words, molecules (e.g., water molecules) or other ions of the liquid solution may migrate between adjacent electrical traces in adjacent single-layer printed circuit boards. That is, an ion migration phenomena may occur between adjacent electrical traces in adjacent single-layer printed circuit boards. As a result, the molecules (e.g., water molecules) or other ions may react with or electrical connect the adjacent electrical traces, as a result, a quality of the electrical traces may be affected, e.g., causing an open circuit phenomena or a short circuit phenomena.
What is need, therefore, is a multilayer printed circuit board which can overcome the above-described problems.